The present invention relates to a steam turbine provided with moving blades having twisted blades twisted about their longitudinal axes and, more particularly, to a steam turbine for use in a thermal or nuclear power plant.
Generally, moving blades included in a steam turbine are caused to vibrate constantly at frequencies in a wide frequency range by streams and their turbulent components of the working fluid (steam). The vibratory response of a blade structure to those excitations is greatly dependent on the respective natural frequencies in each mode of vibration and the magnitude of damping force.
Connecting members called integral covers or integral shrouds are disposed on the tips of the blades, and the connecting members disposed on the tips of the adjacent blades are connected by the blade untwisting effect of centrifugal force that acts on the moving blades when the turbine rotor rotates to bind the tips of the moving blades, because additional effects in enhancing the rigidity of the blade structure and damping vibrations can be expected from the binding of the tips of the moving blades. Thus, resonance in a low-order vibration mode in which resonance response is high can be suppressed and the reliability concerning resonance in a high-order vibration mode in which resonance response is low can be improved.
A moving blade as long as 32 in. or above, such as the moving blade of the last stage in the low-pressure section of a steam turbine, vibrates in a large amplitude. Consequently, an excessively high local stress is induced around a connecting portion of the tip portion of the moving blade or in a base portion of the moving blade, and the stressed portion is damaged. Connecting members called tie bosses or integral snubbers are disposed on middle portions of the front and the back side of each of blades, and the connecting members disposed on the middle portions of the adjacent blades are connected by using the untwisting effect to bind the middle portions of the blades in addition to binding the tips of the blades for relieving stress concentration and suppressing excessive stress generation.
A known technique disclosed in JP-A No. Hei 4-5402 provides moving blades provided with integral shrouds disposed on the tips of blades with the adjacent integral shrouds in surface-contact with each other, and integral snubbers having a cut angle substantially equal to that of the contact surfaces of the integral shrouds and disposed on the front and the back sides of substantially middle portions of the blades. The integral snubbers disposed on the adjacent blades are brought into contact with each other by the untwisting effect of centrifugal force generated when the turbine wheel rotates.
In the moving blade provided with the connecting members on its tip portion and its middle portion, reaction force or pressure (reaction force per unit area) acting on the contact surface of the connecting members disposed on the tip portion, and reaction force or pressure acting on the contact surface of the connecting members disposed on the middle portion of the blade are not determined independently of the rotating speed of the rotor but they are related with each other. In order to control the reaction force or pressure of tip and middle portions to an an allowable range or below, the relation between the respective contact states of the tip portion and the middle portion, i.e., the relation between the respective shapes or types of construction of the contact surface of the tip portion and that of the middle portion, and the relation between the time of contact of the tip portion and that of the middle portion must be taken into consideration.
However, the invention disclosed in Japanese Patent Laid-open No. Hei 4-5402 does not give any consideration to the relation between the respective contact states of the tip portion and the middle portion, which may be because an object of the invention disclosed in the cited reference is simply the extinction of vibrations of secondary mode.
An object of the present invention is to provide a steam turbine provided with moving blades designed taking the relation between the respective contact states of a tip portion and a middle portion of each moving blade into consideration so as to suppress the induction of excessive stresses in the joint of the connecting member and a connecting portion of each moving blade, and having improved reliability in strength and vibration in the operation range from the start of the turbine to the rated operation thereof.
With the foregoing object in view, according to a first aspect of the present invention, a steam turbine comprises a plurality of twisted blades arranged along a direction in which a rotor rotates and twisted about their longitudinal axes, first connecting members formed in a tip portion of each blade so as to extend on back and front sides of the blade, and second connecting members disposed on back and front sides of a middle portion of each blade between a base portion and the first connecting members of each blade; wherein width of a gap along the direction of rotation of the rotor between opposite end surfaces of the first members of the adjacent blades is smaller than width of a gap along the direction of rotation of the rotor between opposite end surfaces of the second members of the adjacent blades.
According to a second aspect of the present invention, a steam turbine comprises a plurality of twisted blades arranged along a direction in which a rotor rotates and twisted about their longitudinal axes, first connecting members formed in a tip portion of each blade so as to extend on back and front sides of the blade, and second connecting members disposed on back and front sides of a middle portion of each blade between a base portion and the first connecting members of each blade; wherein width of a gap along the direction of rotation of the rotor between opposite end surfaces of the first members of the adjacent blades, and width of a gap along the direction of rotation of the rotor between opposite end surfaces of the second members of the adjacent blades are determined so that a rotating speed of the rotor at which the first connecting members of the adjacent blades come into contact is lower than a rotating speed of the rotor at which the second members of the adjacent blades come into contact.
According to a third aspect of the present invention, a steam turbine comprises a plurality of twisted blades arranged along a direction in which a rotor rotates and twisted about their longitudinal axes, first connecting members formed in a tip portion of each blade so as to extend on back and front sides of the blade, and second connecting members disposed on back and front sides of a middle portion of each blade between a base portion and the first connecting members of each blade; wherein width of a gap along the direction of rotation of the rotor between opposite end surfaces of the first members of the adjacent blades, and width of a gap along the direction of rotation of the rotor between opposite end surfaces of the second members of the adjacent blades are determined so that the frequency of natural vibration generated when the rotor rotates changes at a rotating speed not higher than the rated rotating speed of the rotor.